I do use white vinegar as pH down, there's no problem doing that as far as I read, and it's cheap/natural and maybe organic if you find one.
As you are using GH you may make the whole week water solution and let it in a dark place not to get algae. Hydro nutes may stay 7~10 days without problem depending on the quality of your water/ reservoir cleanness.
Remember that you must put the nutes in the order, and wait some minutes between each one introduced. The pH of the solution must be balanced 1 hour after the mix, to pH to stabilize. And I suggest you to don't add too much ph down, just a few drops and wait another hour to measure again. pH some times take minutes/hours to stabilize, and my get another values after some days.
And calibrate you ph Pen, use your tap water as reference to check if it's stable. Mine must be calibrated every time I use.